The present invention relates to an apparatus and a method for detecting a focus condition of an optical system such as a camera, a microscope or the like.
Two methods have been developed for detecting an in-focused condition of an object image formed by an imaging optical system, one being an image sharpness detection method and the other being a lateral shift detection method. FIG. 1 shows an example of applying a focus condition detecting device for carrying out such an image sharpness detection method to a single-lens reflex camera. In FIG. 1, a part or whole of a light flux transmitted through an imaging lens 1 is divided into two parts by a quick-return mirror 3 having a part thereof formed as a half-mirror 2. One of the divided light fluxes is directed to a finder system including a focusing screen 4, a penta prism 5 or the like, and the other part thereof is directed to a beam splitter 7 by a total reflection mirror 6 which is arranged behind the quick-return mirror 3, after transmitting through the half mirror 2. The beam splitter 7 furthermore divides received light flux into two parts and respective subdivided light fluxes are imaged upon two light receiving element arrays 9a and 9b which are disposed at positions separated by a constant distance on both sides of a plane conjugated with a predetermined focal plane 8 (film plane) of the imaging lens 1.
In the construction thus formed, assuming that an output of one light receiving element array is X.sub.n, the following equation may be obtained: EQU S=.vertline.X.sub.n -X.sub.n-1 .vertline..sub.MAX +.vertline.X.sub.n -X.sub.n-1 .vertline..sub.SUBMAX
wherein S is an evaluation value relating to a sharpness of image which varies in accordance with the amount of sharpness thereof.
For outputs of two light receiving element arrays 9a and 9b, assuming that evaluation values S obtained by the above equation are S.sub.1 and S.sub.2, respectively, evaluation values S.sub.1 and S.sub.2 are changed as shown in FIG. 2 for defocusing. When the difference between the values S.sub.1 and S.sub.2 is observed, the defocusing direction and the in-focused position can be detected as a forwardly-defocused condition for S.sub.1 &lt;S.sub.2, a backwardly-defocused condition for S.sub.1 &gt;S.sub.2 and an in-focused condition for S.sub.1 =S.sub.2.
The above described conventional method is capable of detecting in-focused condition with high precision by utilizing a comparatively simple optical system. As is shown in FIG. 2, however, in the condition of largely shifting the imaging plane of the imaging lens 1 from a predetermined in-focused position, the difference between evaluation values S.sub.1 and S.sub.2 becomes very small and then it is difficult to compare both evaluation values S.sub.1 and S.sub.2 with each other, so that it is possible to detect the in-focused condition, the forwardly-defocused condition and the backwardly-defocused condition only in a limited lens drive range, but it is very difficult to detect the in-focused condition over the whole imaging optical system drive range.
An example of the conventional lateral shift detection method is shown in FIG. 3, in which like elements are designated by the same reference characters as that shown in FIG. 1. In an in-focused condition detecting device for carrying out the lateral shift detection method, a light flux of an object (not shown) reflected by the total reflection lens 6 through the imaging lens 1 and the half mirror 2 of the quick-return mirror 3 is incident upon a light receiving element array 11 disposed at a plane substantially optically conjugated with an exit pupil plane of the imaging lens 1 through an auxiliary optical system 10 such as a lenticular lens or the like disposed at a plane conjugated with or near thereof the predetermined focal plane of the lens 1 i.e., the film plane 8. As shown in FIG. 4 the light receiving element array 11 comprises first and second light receiving element groups 11A and 11B each having light receiving elements 11A-1 to 11A-n and 11B-1 to 11B-n, respectively, and respective corresponding light receiving elements constitute light receiving element pairs 11A-1, 11B-1; . . . ; 11A-n, 11B-n. Whole light receiving elements are arranged to position them in a straight line. The auxiliary optical system 10 comprises n auxiliary optical elements corresponding to light receiving element pairs 11A-1, 11B-1; . . . ; 11A-n, 11B-n, and two light receiving elements forming respective light receiving element pairs are so arranged that they receive images transmitted through portions of the exit pupil plane of the imaging lens 1 which are placed at either side of a plane including the optical axis of the imaging lens 1 and perpendicular to an array direction of light receiving element (a plane perpendicular to the paper and including an optical axis in FIG. 3), that is, upper and lower parts of the exit pupil plane which are defined by the optical axis in FIG. 3.
In the construction thus formed as shown in FIG. 3 when at least a part of an object image is incident upon the light receiving element array 11 through the imaging lens 1 and the auxiliary optical system 10, the light receiving element group 11A receives only light flux transmitted through the lower part of the imaging lens 1 and the light receiving element group 11B receives only light flux transmitted through the upper part of the lens 1 so that illumination distributions of the image incident upon the light receiving element groups 11A and 11B coincide with each other in the in-focused condition and shifts laterally in the opposite directions in accordance with the shifting directions in the defocused condition. In the focus condition detecting device shown in FIG. 3, outputs of light receiving element groups 11A and 11B are so processed that lateral shift direction of the image is detected and thus respective focus conditions such as the forwardly- and backwardly-defocused conditions and the in-focused condition are detected based thereon.
Such a focus condition detecting device according to the lateral shift detection method has the advantage that a possible focus condition detecting range becomes larger than that of the image sharpness detection method shown in FIG. 1. On the other hand, a gain of lateral shift signal becomes small near in the in-focused position and the construction thereof becomes complex so that it is difficult to manufacture the auxiliary optical system 10 such as the lenticular lens or the like in the conventional focus condition detecting device shown in FIG. 3 and thus the whole device becomes expensive and large resulting in difficulty in optically adjusting the respective auxiliary optical system and the light receiving element pairs corresponding thereto.